Acetic acid bacteria are microorganisms broadly utilized for vinegar production. In particular, acetic acid bacteria belonging to the genus Acetobacter and the same belonging to the genus Gluconacetobacter are used for industrial acetic acid fermentation.
In acetic acid fermentation, ethanol contained in media is oxidized and converted into acetic acid by acetic acid bacteria. As a result, acetic acid is accumulated in the media. Acetic acid is also inhibitory on acetic acid bacteria. As the amount of accumulated acetic acid increases and the acetic acid concentration in media becomes higher, the growth ability and the fermentation ability of acetic acid bacteria gradually decrease.
In particular, growth induction period, that is, the period until acetic acid bacteria actually start to grow, and then it becomes possible to confirm the accumulation of acetic acid, tends to be longer as the acetic acid concentration becomes higher.
Hence, in acetic acid fermentation, it is desired to further shorten the growth induction period, even in the case of a higher acetic acid concentration. As a means for this purpose, a method has been disclosed that involves adding PQQ (4,5-dihydro-4,5-dioxo-1H-pyrrolo[2,3-f]quinoline-2,7,9-tricarboxylic acid) to a fermention liquid to promote growth, so as to shorten so-called the growth induction period (e.g., see JP Patent Publication (Kokai) No. 61-58584 A (1986)).
However, obtaining PQQ in large quantities is difficult, and PQQ is expensive. Thus, implementation of such a method at industrial scale has been considered to be uneconomical. Accordingly, efforts have been made to breed and improve acetic acid bacteria by promoting the growth (resistance to acetic acid) of acetic acid bacteria in the presence of a high acetic acid concentration, cloning genes encoding proteins having a function capable of shortening so-called the growth induction period (genes involved in growth promotion), and using the genes involved in growth promotion.
However, no genes involved in growth promotion of acetic acid bacteria have been isolated so far. Under such circumstances, isolation of a novel gene having a growth-promoting function and encoding a protein that has functions to promote at a practical level the growth (resistance to acetic acid) of acetic acid bacteria in the presence of a high acetic acid concentration and to shorten the growth induction period, and generating acetic acid bacteria having a stronger growth function using the gene involved in growth promotion have been desired.